Every three minutes, a patient attends a UK hospital after suffering a head injury. Traditionally, the much greater proportion of these patients were male.
This picture seems to be changing, however, according to figures collated by Headway, the UK brain injury association, which shows a 24 per cent increase in hospital admissions among females since 2005.
The reason for this increase is not fully understood. Certainly, the last decade has seen an increase in female participation in contact sports such as football and rugby and sports such as cycling, which also present elevated head injury risk. But the connection remains hypothetical, without a body of research evidence.
Where better understood is how concussion seems to affect women differently to men. Women appear to be more at risk of concussion, more likely to experience the most severe effects of concussion and take longer to recover from concussion, studies have shown.
Tracey Covassin of Michigan State University in the US has been a leading researcher in the field of female concussion, finding in ball sports, females were almost twice as likely to suffer a concussion as male players. Different symptoms were reported: male concussions were more likely to produce amnesia, while females reported prolonged headaches, mental fatigue and difficulties with concentration and mood changes.
Another study of young sportspeople found on average, females took 76 days to recover from a concussion, compared with 50 days for males. Another study has shown differences in cognitive impairment measured after a concussion.
Three theories have been put forward to explain these gender differences. One focuses on the physiology of female necks, emphasising the way injuries can be caused as much by the sudden jerking of the head as much as being the impact of a specific blow. As such, male necks, with a wider average circumference and muscle density are better able to mitigate sudden force and its effect than female necks.
The second theory focuses on how female brains are thought to have slightly faster metabolisms than male brains, with greater blood flow to the head. Therefore, if a head injury momentarily disrupts that supply of glucose and oxygen, it has the potential to cause greater damage.
The third theory considers different hormone levels during the menstrual cycle. One study found injuries during the follicular phase (after menstruation and before ovulation) were less associated with symptoms enduring beyond a month, while an injury during the luteal phase (after ovulation and before menstruation) resulted in more severe and lasting symptoms.
Head injuries can temporarily disrupt the production of various hormones, including progesterone. During the luteal phase progesterone levels are highest, and the researchers suggest the sudden dramatic fall in progesterone due to head injury throws the brain off balance and contributes to the worse and longer lasting symptoms. In the follicular phase, by contrast, progesterone levels are already lower; the drop in progesterone is therefore less dramatic.
There have been some concerns about the effect of emphasising gender differences in concussion: whether this might contribute to male sportsmen feeling their concussion is less serious, exacerbating male tendencies to underplay symptoms and return to activities too soon. Equally, there have been concerns about how differences could work against female participation in sport more generally, which has many recognised health benefits.
The American football player Brittni Souder, who retired from the sport due to a number of concussions, seems to strike a positive balance, working with young players to raise awareness of concussion for females and how to mitigate risks, while also advocating for participation in football.